Detect prior install with .installed and not $SOURCE_DIR/.install.data

Before this change, after installation there was both
$SOURCE_DIR/.install.data and $INSTALL_DIR/.install.data, and both
potentially containing sensitive information like passwords. The only
purpose of the former was to detect whether a Critic installation had
been done earlier from $SOURCE_DIR or not (install.py would then
suggest using upgrade.py instead) and no actual data needed to be in
the file. This change introduces a new, empty, file called
$SOURCE_DIR/.installed which will be used instead to abort install.py
and suggest upgrade.py.
Don't let install.py leave root:root owned file in installation source dir
Clean up root owned pyc files in source dir after upgrade
Restrict permissions for .install.data since it may contain passwords